The invention relates to an optical scanning device with a first deflecting mirror arranged behind a lens and being controllably pivotable as well as at least one further, fixed deflecting mirror arranged in the light path of the first deflecting mirror to project on an exposure area parts of an image that change according to the pivoting motion of the first deflecting mirror. The invention also relates to a microfilm reader-printer with such a device.
By optical scanning devices an image is projected with a lens on a plane exposure area in order to expose for example the base material for the printing of an image. In the case of microfilm reader-printers two scanning principles have proved particularly successful. According to the so-called translational principle a first deflecting mirror moves in a straight-lined manner in the light path of the lens and thus projects an image that moves accordingly in a straiight-lined manner. The base material that is to be exposed has to move in a straight-lined manner according to the advance of the projected image in order to produce a complete and sharp image. The disadvantage of this translational principle that is also generally used for copying machines in offices is that the machine has to be relatively large because of the advance of the base material. This is a particular disadvantage in the case of microfilm reader-printers.
For this rason one often uses the so-called rotational principle for microfilm reader-printers. According to this principle a deflecting mirror pivots evenly in the light path of the lens and thus projects in succession the image that is to be scanned on one and the same area so that the base material that is to be exposed only has to move in that area. As a result the machines that work according to this principle are much smaller than the ones mentioned before. However, the prints they produce are not so exact as concerns the geometry of the images because the distances from the scanned image area via the deflecting mirrors to the exposure area with the base material to be exposed are always the same whereas usual lenses are optically corected so that they project a complete image on a plane area in a geometrically faultless manner in spite of different distances.
The so-called rotational principle thus causes a barrel-shaped distortion that means the width of the image is correct in the middle but at the top and at the bottom the width is reduced because of the curving mistake. But different lenses (often exchangeable up to three) in practice often show different individual distortions so that the distortions of the image have to be individually corrected according to the lens used.